On cigarette packing machines, groups of cigarettes, each defining the content of a packet of cigarettes, are fed successively to a packing line by means of a step-operated supply device comprising a number of pockets equally spaced along the device and each for housing a respective group of cigarettes.
The cigarettes are fed to the supply device by means of a hopper having an output end divided into a number of outlets normally equal to the number of layers defining each group of cigarettes. As each group of cigarettes normally comprises three layers, the hopper normally comprises three outlets, each of which is divided into a number of substantially vertical channels equal to the number of cigarettes in the respective layer.
In the above case, the supply device is fed forward in steps so that, at each stop, three pockets are positioned at the three outlets beneath the output end of the hopper. The output end of the hopper comprises three push members located at increasing levels and activated simultaneously to transfer, at each stop of the supply device, a layer of cigarettes into each of the three pockets facing the push members. Consequently, each step of the supply device corresponds to the formation of one complete group of cigarettes comprising three superimposed layers and housed inside the pocket emerging from beneath the hopper at that particular step.
Though highly efficient, a major drawback of the above known forming device is the formation, when the device is started up and stopped, of incomplete groups of cigarettes which must therefore be rejected.
That is, when the forming device is started up, the three push members, which are activated simultaneously, are positioned facing respective empty pockets, so that, while the first push member may safely feed a respective bottom layer into the facing pocket, the second push member feeds an intermediate layer into a pocket with no bottom layer, and the third push member feeds a top layer into a pocket with no bottom or intermediate layer. Similarly, when the push members are stopped simultaneously, the first and the second group fed out of the hopper by the supply device have, respectively, no top layer and no intermediate or top layer.
The above drawback is at least partly eliminated by U.S. Pat. No. 4,607,475, in which each hopper outlet is assigned a respective intercepting member for intercepting and cutting off supply of the cigarettes to the respective outlet, and which is activated independently of the respective push member. Each intercepting member comprises a flexible diaphragm movable between a rest position, in which the diaphragm is wound about a respective winding roller to the side of the respective outlet, and a work position in which, by means of an electric motor, the diaphragm is unwound off the respective roller and inserted inside a straight guide between the respective push member and respective outlet.
An intercepting member of the above type has serious drawbacks, both technically and in terms of cost.
Firstly, the electric motor for activating the intercepting member complicates the structure of the member itself and increases the overall cost of the forming device, particularly in the case of a hopper with more than three outlets and, hence, more than three intercepting members.
Secondly, being flexible, the intercepting member is subject to jamming when switching from the rest position, in which the flexible diaphragm is curved by being wound about the respective roller, to the work position in which the diaphragm is laid flat along the respective straight guide.